New storage concepts have been introduced over the past few years. Exploiting the capability of imaging and investigating the structure of materials down to the atomic scale achieved by scanning tunneling microscopy (STM) and atomic force microscopy (AFM), probes having a tip are being introduced for scanning appropriate storage media, where data are written as sequences of bits represented by indentations and non-indentations. According to latest demonstrations, indentations with a diameter of the range of 30-40 nm have been written on appropriate storage media. These data storage concepts promise ultra-high storage areal densities.
First approaches are disclosed in “High-density data storage using proximal probe techniques” by H. J. Mamin et al., IBM Journal Research Development, Vol. 39, No. 6, November 1995. A single tip of an AFM cantilever is placed in contact with a rotating surface of a polycarbonate storage medium. Bits are represented by indentations or non-indentations written on the storage medium. Writing on the storage medium is accomplished by heating the tip with a pulsed infrared laser. With the tip being in contact with the storage medium, the heated tip softens the polymer surface. As a force is applied to bring the tip in contact with the surface, the tip creates a small indentation. A mechanical reading mechanism is adopted. As the tip rides over the surface of the storage medium, a topographic indentation causes a deflection of the cantilever. This deflection is detected using a standard optical sensor.
“High-Density Data Storage Based on the Atomic Force Microscope”, by H. J. Mamin et al., Proceedings of the IEEE, Vol. 87, No. 6, June 1999, discloses another single tip based storage device with a rotating disk as storage medium. A single tip at the end of an AFM cantilever is placed in contact with a rotating surface of a polycarbonate storage medium. Bits are represented by indentations or non-indentations in the storage medium. Writing is accomplished by heating the tip electrically via two conducting legs, which are connected with the tip. Reading is accomplished with a piezoresistive sensor, sensing the deflection of the cantilever when scanning an indentation.
Applicant's U.S. Pat. No. 5,835,477 discloses a storage device with a recommendation for rewriting such a storage device. The storage device comprises a circuit for distinguishing between information which is to be erased from a first section of the storage medium and information which is not to be erased in this section. The information not to be erased is copied into another section of the storage device. Afterwards, the first section can be erased.
“The Millipede—More than one thousand tips for future AFM data storage” by P. Vettiger et al., IBM Journal Research Development, Vol. 44, No. 3, May 2000, shows a data storage device based on a mechanical x-/y-scanning of a storage medium with an array of probes each having a tip. The probes are scanning assigned fields of the storage medium in parallel, so high data rates can be achieved. The storage medium comprises a thin polymethylmethacrylate (PMMA) layer. The probes are scanning the polymer layer in a contact mode. The contact mode is achieved by applying small forces to the probes so that the tips of the probes can touch the surface of the storage medium. Therefore spring cantilevers are carrying the sharp tips on their end section. Bits are represented by indentations or non-indentations in the polymer layer. The cantilevers respond to these topographic changes in the surface.
Indentations are written on the polymer surface by thermomechanical recording, whereas the local probe is heated with a current or voltage pulse during the contact mode, so that the polymer layer is softened locally where the tip touches the polymer layer. The result is a small indentation in the layer, having nanoscale diameter. Reading is also accomplished by a thermomechanical concept. The heater cantilever originally used only for writing is given an additional function of a thermal reading sensor by virtue of its temperature dependent resistance. For reading purposes, the resistor is operated at a temperature that is not high enough to soften the polymer as is necessary for writing. The thermal sensing is based on the fact that the thermal conductance between the probe and the storage substrate changes when the probe is moving into an indentation, as the heat transport will be more efficient. Consequently the heater's temperature and hence its resistance will decrease. Thus, changes of the continuously heated resistor are monitored while the cantilever is scanned over a corresponsing data field.
For reasons of power conservation, periodic current or voltage pulses of short duration, rather than DC current or DC voltage, are applied to the cantilevers in order to heat the cantilever to the appropriate temperature for reading purposes. In addition, applying a DC current or a DC voltage would increase the heat transferred to the storage medium and reduce its average lifetime.
Since the storage medium is moved relatively to the probes with a certain velocity, every T seconds a reading pulse has to be fired to the probes for mark and therefore data detecting purposes. As used herein “mark” is understood as physical representation of an information unit. Referring to prior art storage devices, marks are indentations and non-indentations for instance. T corresponds to the time it takes for a probe to scan the distance between the center of two consecutive marks at a given scanning velocity. The duration of such reading pulses is small compared to the time it takes for a probe to scan over a mark. Accurate timing of the pulses becomes critical, since the amplitude of a response signal caused by a reading pulse drops as the probe moves away from the center of an indentation mark. As a result optimum data detection is obtained if reading pulses are fired whenever the probe is in a position corresponding to the center of a mark. By “optimum data detection” is meant that the probability of detecting a bit corresponding to a certain value, given that a bit corresponding to the opposite value has been stored, is minimized. But even firing a pulse at the right time once, and therefore showing an accurate clock phase to meet the center of a mark, is no guarantee for meeting the centers of following marks with succeeding pulses. This may be due to variations of the scanning velocity or to variations of the clock phase or frequency.
Accordingly, it is a challenge to provide a nanoscale storage device and a method for operating the storage device with an accurate timing for firing reading pulses in a scanning mode.